Overall Project The long-term objective of this Program Project is to perform a systematic analysis of the cellular and molecular mechanisms regulating thymic epithelial cell (TEC) homeostasis and the role of TECs in the involution of the adult thymus. The thymus is the primary lymphoid organ responsible for the generation of functional T cells, and is therefore critical for generation and maintenance of adaptive immunity. It is also one of the first organs to undergo significant age- related organ degeneration, termed thymic involution. Thymic involution results in a dramatic drop in the production of new T cells, and is a significant contributing factor in immune senescence. Despite the importance of this subject for human health, the molecular mechanisms that operate in the postnatal thymus and mediate thymic homeostasis and involution are largely unknown. As a result, many aspects of thymic involution are poorly understood and/or controversial. Understanding the relative contributions of the mechanisms underlying this process would have significant implications for improving human health. The proposed Program Project is designed to identify mechanisms and pathways that can be exploited for induction of thymic rebound and/or prevention of involution, focused on the role of changes in TECs. The two projects will test two central hypotheses: 1)that failure to maintain postnatal thymic epithelial cell homeostasis is a causative mechanism in thymic involution;and 2) that down regulation of Foxnl, a pivotal regulator of TEC development and maintenance, is a primary molecular mechanism in thymic involution. These hypotheses will be addressed through three major Program objectives which will investigate: a) the lineage relationships of TEC subsets in the postnatal thymus;b) the molecular mechanisms driving TEC homeostasis;and c) whether these mechanisms are causal for involution. The scientific projects share the common goal of identifying the cellular and molecular mechanisms resulting in the failure of TEC maintenance in thymic involution. The approaches used are all grounded in the central idea that the mechanisms that regulate fetal thymus development and TEC differentiation provide a conceptual framework for understanding thymic aging and regeneration. The individual projects in the P01 will take overlapping and complementary approaches to test these hypotheses that combine the unique expertise, experimental tools, and institutional resources of the investigators.